The present invention relates to gas laser optics and specifically to optics for achieving a folded laser cavity.
It is common practice in gas laser technology to achieve fundamental mode operation (TEM.sub.00) by choosing the laser bore diameter and spherical mirror radia such that the higher order modes have diffraction loses in excess of the laser gain. This technique favors a relatively narrow bore diameter. However, in order to increase power output of the laser without making the bore diameter excessively large, the length of the tube must be increased. In many applications, it is desirable to have as short a laser tube as possible and yet maximize the power output of the device.
Another consideration when making very short laser tubes is the longitudinal mode separation with respect to the doppler width of the output. The longitudinal mode separation is defined as c/2L where c is the speed of light and L is the length of the laser tube. In order to produce a gas laser having good output stability without increasing the cost of the device considerably, the longitudinal mode separation must be less than the doppler width. However, in designing relatively short laser tubes it is difficult to achieve this end. Heretofore in solid state lasers, folded optical cavities have been used to decrease the length of the laser. A folded optical cavity is one in which there are two or more light paths through the lasing medium as shown in U.S. Pat. No. 3,482,186 issued on Dec. 2, 1969 to J. P. Chernoch. However, these devices required complex alignment of three mirrors and several mode selection apertures.